In screen printing operations, a porous screen formed of any of various fabrics such as silk, polyester or of metal mesh is fastened to a screen frame. A negative of the image which is to be printed may be painted on the screen or a stencil like reproduction of the image may be adhered to the screen. The material which is to receive the image is disposed on a platen or work table and the screen frame is impressed against the material. A squeegee is then used to force paste-like ink of the desired color through the unpainted areas of the screen or through the open areas of the stencil. The frame is then retracted leaving the desired image on the imprinted material. Additional impressions on the same area of the material can be made with additional screens when a multi-colored image is desired.
Screen printing is used extensively for printing works of art designs, or information on materials which may be of any of a variety of types such as posters, signs, clothing or glass or metal objects among other examples. Screen printing is advantageous for many purposes as the required equipment can be less costly than other forms of printing apparatus and set-up procedures can be relatively simple.
In most operations, it is desirable that the screen frame register precisely with a predetermined area of the platen which supports the material to be imprinted. This enables the printed image to be placed at exactly the same location on successive pieces of material. Precise registration is particularly important when a series of screen frames must be impressed on the same piece of material to provide a multi-color image or for other reasons. An even minute displacement of one color relative to other colors in the image can significantly detract from image quality.
Screen printing presses designed for high volume production typically have a screen frame clamping assembly or a platen assembly which is pivoted to bring the frame and platen together during a printing operation and to separate the members for removal of the printed product and insertion of another piece of material. The frame supporting assembly is manufactured with close tolerances to inhibit misregistration but this objective is not always realized to the most desirable extent. Significant looseness or play in the frame clamping arm is often encountered and tends to become more pronounced after wear has occurred.
Registering mechanisms relying on fixed register pins on one of two mating members that enter fixed passages on the other member have heretofore been used in some types of printing apparatus but are not entirely satisfactory in the case of screen printing presses of the type described above. In addition to being complicated and costly, such pin registration mechanisms complicate set up procedures. The prior mechanisms would not, for example, enable shifting of the screen frame to register with a selected area of the platen at a time when a pin is already engaged in its mating passage which capability could facilitate initial set-up of a press. Prior pin registration mechanisms are also not readily adaptable to the use of screens of different sizes or configurations.
The present invention is directed to overcoming one or more of the problems discussed above.